A blood gas is exactly that...it measures the dissolved gases in your bloodstream. This provides one of the best measurements of what is known as the acid-base balance. The body is an active chemical reaction, requiring a precise balance of the acids and bases, a measurement known as pH. The normal pH is 7.40. There are four parameters which are measured:
Carbon Dioxide (CO2)
The two main organs regulating the acid-base balance are the lungs and kidneys. Looking at the gases that are measured, it is easy to see how the lung is involved, bringing in oxygen and expelling carbon dioxide. This is known as respiratory metabolism. The kidney is important in metabolic control by actively regulating the dissolved CO2 or bicarbonate which is in the plasma. This is expelled through the urine.
The body is able to respond to changes in the pH either immediately through respiratory control or by slowly adapting to it through the kidneys.
Base excess or buffer base (strong ion difference) as measure of a non-respiratory acid-base disturbance.
Siggaard-Andersen O, Fogh-Andersen N.
Department of Clinical Biochemistry, Herlev Hospital, University of Copenhagen, Denmark.
Acta Anaesthesiol Scand Suppl 1995;107:123-8 Abstract quote
Stewart in 1983 (Can J Physiol Pharmacol 1983: 61: 1444) reintroduced plasma buffer base under the name "strong ion difference" (SID). Buffer base was originally introduced by Singer and Hastings in 1948 (Medicine (Baltimore) 1948: 27: 223). Plasma buffer base, which is practically equal to the sum of bicarbonate and albuminate anions, may be increased due to an excess of base or due to an increased albumin concentration. Singer and Hastings did not consider changes in albumin as acid-base disorders and therefore used the base excess, i.e., the actual buffer base minus the buffer base at normal pH and pCO2, as measure of a non-respiratory acid-base disturbance. Stewart and followers, however, consider changes in albumin concentration to be acid-base disturbances: a patient with normal pH, pCO2, and base excess but with increased plasma buffer base due to increased plasma albumin concentration get the diagnoses metabolic (strong ion) alkalosis (because plasma buffer base is increased) combined with metabolic hyperalbuminaemic acidosis. Extrapolating to whole blood, anaemia and polycytaemia should represent types of metabolic alkalosis and acidosis, respectively. This reveals that the Stewart approach is absurd and anachronistic in the sense that an increase or decrease in any anion is interpreted as indicating an excess or deficit of a specific acid. In other words, a return to the archaic definitions of acids and bases as being the same as anions and cations.
We conclude that the acid-base status (the hydrogen ion status) of blood and extracellular fluid is described in terms of the arterial pH, the arterial pCO2, and the extracellular base excess. It is measured with a modern pH-blood gas analyser. The electrolyte status of the plasma is a description of the most important electrolytes, usually measured in venous blood with a dedicated electrolyte analyser, i.e., Na+, Cl-, HCO3-, and K+. Albumin anions contribute significantly to the anions, but calculation requires measurement of pH in addition to albumin and is usually irrelevant. The bicarbonate concentration may be used as a screening parameter of a nonrespiratory acid-base disturbance when respiratory disturbances are taken into account. A disturbance in the hydrogen ion status automatically involves a disturbance in the electrolyte status, whereas the opposite need not be the case.
The disequilibrium pH: a tool for the localization of carbonic anhydrase.
Division of Environmental and Evolutionary Biology, University of Glasgow, Scotland.
Comp Biochem Physiol A Mol Integr Physiol 1998 Jan;119(1):243-54 Abstract quote
The disequilibrium pH is defined as any discrepancy between the measured pH and the pH which would exist if CO2-HCO3-H+ reactions were at equilibrium.
Measurement of the disequilibrium pH can be used to assess the status of CO2-HCO3--H+ reactions and, in combination with carbonic anhydrase (CA) or CA inhibitor treatments, may also be used to localize CA. Renal physiologists have used disequilibrium experiments to determine that HCO3- reabsorption in the kidney tubule occurs via proton secretion, and that CA activity is available to ultrafiltrate CO2-HCO3-H+ reactions in the proximal convoluted tubule, but not the distal tubule. Disequilibrium experiments were also used in investigating the availability of CA to CO2-HCO3--H+ reactions in water at the fish gill; the opposing results obtained in two studies have not yet been resolved. Respiratory physiologists have used the disequilibrium technique in vivo and with saline-perfused preparations to assess the availability of CA to plasma CO2-HCO3--H+ reactions following gas exchange. Saline-perfused preparations enable direct localization of CA activity, while in vivo measurements encompass the numerous factors affecting CO2-HCO3--H+ equilibration in a multi-phase solution.
Given the many organs in which membrane-bound CA activity has now been identified, the usefulness of the disequilibrium pH technique has increased beyond its original applications in renal and pulmonary physiology.
Adrogue HE, Adrogue HJ.
Department of Medicine, University of Minnesota, Minneapolis, USA
Respir Care 2001 Apr;46(4):328-41 Abstract quote
Acid-base homeostasis involves chemical and physiologic processes responsible for the maintenance of the acidity of body fluids at levels that allow optimal function of the whole individual.
The chemical processes represent the first line of defense to an acid or alkali load and include the extracellular and intracellular buffers, whereas the physiologic processes modulate acid-base composition by changes in cellular metabolism and by adaptive responses in the excretion of volatile acids by the lungs and fixed acids by the kidneys. The need for the existence of multiple mechanisms involved in acid-base regulation stems from the critical importance of the hydrogen ion (H+) concentration on the operation of many cellular enzymes and function of vital organs, most prominently the brain and the heart. The task imposed on the mechanisms that maintain acid-base homeostasis is large, since metabolic pathways are continuously consuming or producing H+, and the daily load of waste products for excretion in the form of volatile and fixed acids is substantial.
We review the determinants of the acidity of body fluids, the mechanisms that maintain normal acid-base composition, and the overall defense to disruption in acid-base equilibrium. Specific topics include an examination of the scales of acidity, buffer systems, intracellular acid-base regulation, excretion of acids, alkali and acid loading, and normal acid-base composition. The limitations of arterial blood sampling in the assessment of acid-base status are also evaluated.
Continuous intrapartum pH, pO2, pCO2, and SpO2 monitoring.
McNamara HM, Dildy GA
3rd. Department of Obstetrics and Gynecology, McGill University, Canada.
Obstet Gynecol Clin North Am 1999 Dec;26(4):671-93 Abstract quote
The goal of intrapartum surveillance and its further development is better patient care for both the fetus and the gravida. A normal FHR pattern is usually associated with the delivery of a normal well-oxygenated infant; however, a nonreassuring FHR is not always associated with the delivery of a compromised infant. This situation has led to an increase in unnecessary obstetric interventions in the form of a rising cesarean section rate. Fetal scalp sampling was developed in an attempt to improve the predictive value of electronic FHR monitoring, but because this technique is not widely used, management decisions are frequently made using FHR patterns alone. Much research has been performed in the search for a continuous biochemical measurement of fetal status, including continuous pH, pO2, or pCO2 and various combinations of these methodologies. None of these measurements are used in current clinical practice, mainly owing to technical problems and difficulties associated with the continuous direct measurement of these parameters in fetal blood throughout labor.
The promising new field of fetal pulse oximetry has the potential to provide reliable, meaningful, and reproducible data as shown in early cross-sectional studies and more recent longitudinal studies. By identifying developing hypoxia, this technology may reduce the uncertainty associated with electronic FHR monitoring. Fetal pulse oximetry may also provide critical information relating to the detection and management of the hypoxic fetus. Any new method of intrapartum fetal monitoring requires careful evaluation to assess its potential value before its introduction into clinical practice. The use of fetal SpO2 monitoring in the presence of a nonreassuring FHR pattern is being examined in a multicenter randomized controlled trial. This study will address the question of whether supplementary monitoring of fetal SpO2 levels can lead to a reduction in the cesarean section rate for fetal distress. The available data on fetal noninvasive pulse oximetry have been obtained from a combination of well-designed cohort studies (level II-2 evidence) or from earlier multiple time series (level II-3 evidence).
The results from the US Multicenter Trial (level I evidence) should provide a significant addition to current evidence. A continuous fetal noninvasive monitor measuring fetal oxygenation directly could lead to an improvement in the sensitivity and specificity of fetal surveillance. This improvement could ultimately result in a reduction in unnecessary interventions by differentiating hypoxic fetuses from nonhypoxic fetuses and, more importantly, may lead to earlier intervention for fetuses in danger of serious compromise.
Is acid base determination an accurate predictor of pyloric stenosis?
Oakley EA, Barnett PL.
Department of Emergency Medicine, Royal Children's Hospital, Parkville, Victoria, Australia.
J Paediatr Child Health 2000 Dec;36(6):587-9 Abstract quote
OBJECTIVE: To determine if acid base status predicts which vomiting patients have pyloric stenosis.
DESIGN: Retrospective chart review.
SETTING: Tertiary paediatric hospital.
METHODOLOGY: We compared the clinical and biochemical parameters of 100 patients with a discharge diagnosis of pyloric stenosis and 84 patients of a similar age who presented to the emergency department with vomiting and who had an acid base determination. Patients were included from January 1995 to January 1997. Clinical correlates consisted of age, duration of vomiting, weight loss, gestation, and family history of pyloric stenosis. Biochemical correlates were pH, bicarbonate, base excess (BE), chloride, potassium, and sodium.
RESULTS: Independent variables of significance were pH, BE, chloride, bicarbonate, potassium, weight loss (all of which had a P value < 0.0001), and sex (P = 0.006). Each variable was placed in a logistic regression equation with pyloric stenosis being the dominant variable. Variables of significance were pH (P = 0.0001), BE (P = 0.0001), and chloride (P = 0.009). A model for predicting pyloric stenosis using these variables was then created with pH > 7.45, chloride < 98, and BE > +3, with a positive predictive value of 88%.
CONCLUSION: Acid base determination is a useful screening tool when considering pyloric stenosis. This model now needs to be validated on a prospective series of patients with vomiting.
Difference in acid-base state between venous and arterial blood during cardiopulmonary resuscitation.
Weil MH, Rackow EC, Trevino R, Grundler W, Falk JL, Griffel MI.
N Engl J Med 1986 Jul 17;315(3):153-6 Abstract quote
We investigated the acid-base condition of arterial and mixed venous blood during cardiopulmonary resuscitation in 16 critically ill patients who had arterial and pulmonary arterial catheters in place at the time of cardiac arrest.
During cardiopulmonary resuscitation, the arterial blood pH averaged 7.41, whereas the average mixed venous blood pH was 7.15 (P less than 0.001). The mean arterial partial pressure of carbon dioxide (PCO2) was 32 mm Hg, whereas the mixed venous PCO2 was 74 mm Hg (P less than 0.001). In a subgroup of 13 patients in whom blood gases were measured before, as well as during, cardiac arrest, arterial pH, PCO2, and bicarbonate were not significantly changed during arrest. However, mixed venous blood demonstrated striking decreases in pH (P less than 0.001) and increases in PCO2 (P less than 0.004).
We conclude that mixed venous blood most accurately reflects the acid-base state during cardiopulmonary resuscitation, especially the rapid increase in PCO2. Arterial blood does not reflect the marked reduction in mixed venous (and therefore tissue) pH, and thus arterial blood gases may fail as appropriate guides for acid-base management in this emergency.
Assessing acid-base status in circulatory failure. Differences between arterial and central venous blood.
Adrogue HJ, Rashad MN, Gorin AB, Yacoub J, Madias NE.
Department of Medicine, Baylor College of Medicine, Houston.
N Engl J Med 1989 May 18;320(20):1312-6 Abstract quote
To assess arteriovenous differences in acid-base status, we measured the pH and partial pressure of carbon dioxide (PCO2) in blood drawn simultaneously from the arterial and central venous circulations in 26 patients with normal cardiac output, 36 patients with moderate and 5 patients with severe circulatory failure, and 38 patients with cardiac or cardiorespiratory arrest.
The patients with normal cardiac output had the expected arteriovenous differences: venous pH was lower by 0.03 unit, and venous PCO2 was higher by 0.8 kPa (5.7 mm Hg). These differences widened only slightly in those with moderate cardiac failure. Additional simultaneous determinations in mixed venous blood from pulmonary arterial catheters were nearly identical to those in central venous blood. In the five hypotensive patients with severe circulatory failure there were substantial differences between the mean arterial and central venous pH (7.31 vs. 7.21) and PCO2 (5.8 vs. 9.0 kPa [44 vs. 68 mm Hg]). Large arteriovenous differences were present during cardiac arrest in patients whose ventilation was mechanically sustained, whether sodium bicarbonate had been administered (pH, 7.27 vs. 7.07; PCO2, 5.8 vs. 8.6 kPa [44 vs. 65 mm Hg]) or not (pH, 7.36 vs. 7.01; PCO2, 3.7 vs. 10.2 kPa [28 vs. 76 mm Hg]). By contrast, in patients with cardiorespiratory arrest, large arteriovenous differences were noted only when sodium bicarbonate had been given (pH, 7.24 vs. 7.01; PCO2, 9.5 vs. 16.9 kPa [71 vs. 127 mm Hg]).
We conclude that both arterial and central venous blood samples are needed to assess acid-base status in patients with critical hemodynamic compromise. Although information about arterial blood gases is needed to assess pulmonary gas exchange, in the presence of severe hypoperfusion, the hypercapnia and acidemia at the level of the tissues are detected better in central venous blood.
Arterial blood gas changes in panic disorder and lactate-induced panic.
Papp LA, Martinez JM, Klein DF, Ross D, Liebowitz MR, Fyer AJ, Hollander E, Gorman JM.
Anxiety Disorders Clinic, New York State Psychiatric Institute, NY 10032.
Psychiatry Res 1989 May;28(2):171-80 Abstract quote
Lactate infusions were conducted in 12 male panic patients and 8 male normal controls with arterial catheters in place to reassess previously reported acid-base changes based on venous blood samples. The analysis of arterial pH, carbon dioxide pressure, and bicarbonate concentration confirmed most venous findings. At baseline, before the infusion, venous blood shows evidence of mixed chronic and acute respiratory alkalosis in patients while arterial blood gasses are most consistent with developing acute respiratory alkalosis. During the infusion both bloods are consistent with mixed metabolic and respiratory alkalosis with the patients hyperventilating more than normal controls and panicking patients hyperventilating more than nonpanicking patients.
Arterial blood seems more sensitive than venous blood in detecting baseline differences between panicking and nonpanicking patients. A baseline arterial carbon dioxide pressure of 40 mmHg or higher and an arterial pH below 7.40 may predict no subsequent panic to lactate infusion.
Acid base changes in arterial and central venous blood during cardiopulmonary resuscitation.
Steedman DJ, Robertson CE.
Department of Accident & Emergency Medicine, Royal Infirmary, Edinburgh.
Arch Emerg Med 1992 Jun;9(2):169-76 Abstract quote
Twenty-seven patients in cardiopulmonary arrest had simultaneous measurements of arterial and central venous blood gases during cardiopulmonary resuscitation (CPR) with a pneumatic chest comparison and ventilation device.
Mean central venous and arterial hydrogen ion concentrations, PCO2 and calculated bicarbonate concentrations were significantly different (P less than 0.01) at all sampling times (0, 10 and 20 min). Central venous blood samples predominantly showed a respiratory acidosis in contrast to a mixed disturbance in arterial samples inclined towards a metabolic acidosis. The mean difference between central venous PCO2 (pcv CO2) and arterial PCO2 (pa CO2) ranged from 5.18 to 5.83 kPa reflecting the low blood flow in patients undergoing CPR. Measurement of arterial Po2 indicated adequate oxygenation using the pneumatic device.
Arterial blood gas analysis alone does not reflect tissue acid base status. Bicarbonate administration during CPR may have adverse effects and any decision as to its use should be based on central venous blood gas estimations.
Acid-base status of blood from intraosseous and mixed venous sites during prolonged cardiopulmonary resuscitation and drug infusions.
Abdelmoneim T, Kissoon N, Johnson L, Fiallos M, Murphy S.
Department of Pediatrics, University of Florida, USA.
Crit Care Med 1999 Sep;27(9):1923-8 Abstract quote
OBJECTIVES: a) To determine the relationship of acid-base balance (pH, PCO2) of blood samples from the intraosseous and the mixed venous route during prolonged cardiopulmonary resuscitation; b) to compare the effect of separate infusions of epinephrine, fluid boluses, or sodium bicarbonate through the intraosseous sites on the acid-base status of intraosseous and mixed venous blood during cardiopulmonary resuscitation; and c) to compare pH and Pco2 of intraosseous and mixed venous blood samples after sequential infusions of fluid, epinephrine, and sodium bicarbonate through a single intraosseous site.
DESIGN: Prospective, randomized study.
SETTING: Animal laboratory at a university center.
SUBJECTS: Thirty-two mixed-breed piglets (mean weight, 30 kg).
INTERVENTIONS: Piglets were anesthetized and prepared for blood sampling and cardiopulmonary resuscitation. After anoxic cardiac arrest, ventilation was resumed and chest compression was resumed. Blood gas samples from the pulmonary artery and both intraosseous sites were obtained simultaneously at baseline, at cardiac arrest, and at 5, 10, 15, 20, and 30 mins of cardiopulmonary resuscitation for group 1 (control group) and after drug (epinephrine and sodium bicarbonate) and saline infusions via one of the intraosseous cannulas in groups 2 through 5.
MEASUREMENTS AND MAIN RESULTS: We found no differences between intraosseous and mixed venous pH and Pco2 during periods of <15 mins of cardiopulmonary resuscitation. However, this relationship was not maintained during prolonged cardiopulmonary resuscitation and after bicarbonate infusion. After large volume saline infusion, the pH and Pco2 of mixed venous and intraosseous blood were Adv Perit Dial 2001;17:235-7 Related Articles, Books Acid-base balance and nutrition in peritoneal dialysis. Kung SC, Morse SA, Bloom E, Raja RM. Albert Einstein Medical Center, Philadelphia, Pennsylvania, USA. Acidosis has been implicated in increased protein catabolism and malnutrition of dialysis. The present study examines the effect of acid-base balance on the nutrition status of peritoneal dialysis (PD) patients. We followed 43 PD patients for one year. Blood chemistries were measured monthly. Patients were divided on the basis of subjective global assessment (SGA) into well-nourished (A), mildly-to-moderately malnourished (B), and severely malnourished (C) groups. Mean serum bicarbonate and albumin concentrations were, for group A (n = 16), 23.5 mmol/L and 3.96 g/dL respectively; for group B (n = 17), 27.2 mmol/L and 3.50 g/dL respectively; and for group C (n = 10), 25.9 mmol/L and 2.9 g/dL respectively. In group A, mean serum bicarbonate was significantly lower, and albumin concentration significantly higher as compared with the other groups. Interestingly, of 9 patients with serum HCO3 < 22 mmol/L, 6 were in group A and 2 were in group B. Of 6 patients with serum HCO3 > 29 mmol/L, 5 were in group B and 4 were in group C. The data suggest that well-nourished PD patients tend to be more acidotic. Malnutrition in alkalotic PD patients may be due to low protein intake resulting in decreased acid production; however, an effect of alkalosis on protein metabolism cannot be excluded.similar. During epinephrine infusion, the relationship between intraosseous and mixed venous pH and Pco2 was similar to that found in the control group.
CONCLUSIONS: The intraosseous blood sample could be used to assess central acid-base balance in the early stage of arrest and cardiopulmonary resuscitation of <15 mins. However, during cardiopulmonary resuscitation of longer duration, drug infusions may render the intraosseous site inappropriate for judging central acidosis.
Acid-base balance and nutrition in peritoneal dialysis.
Kung SC, Morse SA, Bloom E, Raja RM.
Albert Einstein Medical Center, Philadelphia, Pennsylvania, USA.
Adv Perit Dial 2001;17:235-7 Abstract quote
Acidosis has been implicated in increased protein catabolism and malnutrition of dialysis. The present study examines the effect of acid-base balance on the nutrition status of peritoneal dialysis (PD) patients.
We followed 43 PD patients for one year. Blood chemistries were measured monthly. Patients were divided on the basis of subjective global assessment (SGA) into well-nourished (A), mildly-to-moderately malnourished (B), and severely malnourished (C) groups. Mean serum bicarbonate and albumin concentrations were, for group A (n = 16), 23.5 mmol/L and 3.96 g/dL respectively; for group B (n = 17), 27.2 mmol/L and 3.50 g/dL respectively; and for group C (n = 10), 25.9 mmol/L and 2.9 g/dL respectively. In group A, mean serum bicarbonate was significantly lower, and albumin concentration significantly higher as compared with the other groups. Interestingly, of 9 patients with serum HCO3 < 22 mmol/L, 6 were in group A and 2 were in group B. Of 6 patients with serum HCO3 > 29 mmol/L, 5 were in group B and 4 were in group C.
The data suggest that well-nourished PD patients tend to be more acidotic. Malnutrition in alkalotic PD patients may be due to low protein intake resulting in decreased acid production; however, an effect of alkalosis on protein metabolism cannot be excluded.
Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department.
Kelly AM, McAlpine R, Kyle E.
Department of Emergency Medicine, Western Hospital, Private Bag, Footscray 3011, Melbourne, Australia and the University of Melbourne.
Emerg Med J 2001 Sep;18(5):340-2 Abstract quote
OBJECTIVE:This study aims to determine the extent of correlation of arterial and venous pH with a view to identifying whether venous samples can be used as an alternative to arterial values in the clinical management of selected patients in the emergency department.
METHODS:This prospective study of patients who were deemed by their treating doctor to require an arterial blood gas analysis to determine their ventilatory or acid-base status, compared pH on an arterial and a venous sample taken as close to simultaneously as possible. Data were analysed using Pearson correlation and bias (Bland-Altman) methods.
RESULTS:Two hundred and forty six patients were entered into the study; 196 with acute respiratory disease and 50 with suspected metabolic derangement. The values of pH on arterial and venous samples were highly correlated (r=0.92) with an average difference between the samples of -0.4 units. There was also a high level of agreement between the methods with the 95% limits of agreement being -0.11 to +0.04 units.
CONCLUSION:Venous pH estimation shows a high degree of correlation and agreement with the arterial value, with acceptably narrow 95% limits of agreement. Venous pH estimation is an acceptable substitute for arterial measurement and may reduce risks of complications both for patients and health care workers.
Clinical Diagnosis and Management by Laboratory Methods. 20th Edition. Henry JB. WB Saunders 2001.
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